CHICAGO — President Barack Obama flew into Chicago grimly set on ending the war in Afghanistan — and left his hometown by gleefully starting a new one over Bain Capital with Mitt Romney.

If Obama had any reticence about diving into an election-year controversy at an otherwise deeply sober gathering of world leaders bent on ending 10 years of conflict, he wasn’t showing it Monday when asked about the escalating controversy over Romney’s stewardship of his highly profitable private equity firm.

And not just Romney — Obama’s damn-the-torpedoes remarks were also aimed at some of his fellow Democrats who are increasingly anxious that their party’s leader is attacking a private equity firm for doing what such businesses were created to do: make cash within the confines of the law.

“My opponent, Gov. Romney, his main calling card for why he should be president is his business experience,” Obama told American and international reporters at McCormick Place at the conclusion of the two-day NATO summit focused on the seemingly weightier issues of Afghan withdrawal and Pakistan.

The deep blue of the NATO stage backdrop seemed, for a moment, to morph into the sky blue of Obama’s campaign sets.

“When you’re president, as opposed to the head of a private equity firm, then your job is not simply to maximize profits. … So, if your main argument for how to grow the economy is ‘I knew how to make a lot of money for investors,’ then you’re missing what this job is about,” Obama said. “It doesn’t mean you weren’t good at private equity, but that’s not what my job is as president.”

Obama’s staff and Romney aides have been locked in an intensifying battle after Newark Mayor — and Obama ally — Cory Booker blasted the Obama campaign’s targeting of the venture capital firm as “nauseating” and a “distraction from the real issues.”

Republicans spent the day seizing on Booker’s comments, hoping to drive a clear wedge between Obama and some Democrats. And Romney cited Booker by name in his own response to Obama’s afternoon attack.

“President Obama confirmed today that he will continue his attacks on the free enterprise system, which Mayor Booker and other leading Democrats have spoken out against,” Romney said in a statement. “What this election is about is the 23 million Americans who are still struggling to find work and the millions who have lost their homes and have fallen into poverty. President Obama refuses to accept moral responsibility for his failed policies. My campaign is offering a positive agenda to help America get back to work.”

For Obama’s campaign, Bain Capital remains a potent weapon against Romney. But for some Democrats, it’s a headache they don’t want.

Obama made abundantly clear at his press conference that it’s on the table, whether members of his party like it or not.

“This is not a distraction. This is what this campaign is going to be about,” Obama said. “Both the upsides and the downsides are worth examining.”

Yet comments over the past two weeks from former White House “car czar” Steve Rattner, Booker and former Rep. Harold Ford underscore the different campaign needs of Obama and fundraising needs of his party, which has already seen a drop-off in Wall Street contributions across the board over the past few years.

“It’s a big problem for the party because the Democrats’ fundraising base is shrinking,” said Democratic strategist Dan Gerstein, who advised Joe Lieberman’s Senate run as an independent in 2006. “Old sources are not as reliable, so they are scared to slaughter one of their most productive cash cows. It’s the reason why we had the Glass-Steagall repeal, the housing bubble and no oversight of Wall Street during the run-up to the collapse. But clutching onto that old lifeline is a major danger for Democrats.”

“The country is on to the con and knows that the system has been rigged to favor certain interests,” he added. “The irony, of course, is that if Obama had adopted this critique at the beginning of his term instead of the end, he probably wouldn’t be nearly as weak as he is now. Bailouts would not have been such an albatross around his neck, he would have got more credit for the auto-industry saving, and the tea party never would have got the traction it did in the 2010 cycle.”

Indeed, the perception that Obama is not bowing to the concerns of donor elites ultimately serves the president well. That’s especially true if he can walk a line of his own on criticizing elements of Wall Street and the relevance of private equity to Romney’s résumé while raising from Wall Street donors himself. Obama has run against his party before, and his team tends to prefer it as a brand-strengthening exercise.

But the comments only reinforce the trickle-down effect on the Democratic fundraising brand after years of debate over financial regulatory reform, apathy toward donor maintenance and repeated condemnations of Wall Street excess.

One prominent business official, who asked not to be identified, put it this way: “It’s demonization of capitalism. And that makes a lot of Democrats uncomfortable and Cory Booker’s one of them. … I think that anybody with half a brain knows that the story is far more complicated and, in fact, Bain and private equity generally have made some positive contributions.”

“You know, they’re known as a very good company,” one Democratic operative said of Bain. “[They] help a lot of businesses around the country. Smart Dems don’t want to be anti-business.”

The private-equity community is not a major source of donations for Democrats at this point — but criticism of it has become entangled in perceptions of Wall Street and the business community at large.

Senators such Chuck Schumer and Kirsten Gillibrand, for instance, are among those who still take in hundreds of thousands of dollars from the financial services sector, and would be loath to weigh in.

So will some Democratic governors and other statewide officials, for whom private-equity investments are drivers of state pension funds.

“The governors and the state officials are going to be a lot less comfortable doing [the Bain attacks] because of that relationship,” one prominent Republican operative said. “Whereas if you’re a U.S. senator or a House member, you have” a little more leeway.

The Obama campaign, meanwhile, continued dominating the latest in its string of mini-news cycles with the comments from Booker and then his YouTube walkback of his indictment of the Bain critiques on “Meet the Press.”

One Democratic operative wondered aloud whether “he was sending signals using finger taps or blinks” as Booker delivered a message he appeared to have been pressured into. Obama insiders denied pushing Booker to apologize.

Ford said Monday that he would not have stepped back from Booker’s initial comments.

Democrats consider the Booker and Ford comments to be a sideshow that elevates the cable news chatter level but doesn’t hurt their overall message where they care about it, which is in the Midwest.

If senators in target states were denouncing the Bain message, it would be more of a concern, said a senior Democrat who added that Ford, out of office for years and on TV almost exclusively, “is irrelevant.”

Chris Lehane, Al Gore’s 2000 campaign manager, welcomed the elevation by Chicago of the Booker comments, which Obama adviser David Axelrod denounced as “wrong” on MSNBC. And the president’s own comments guarantee that the storyline lasts another day.

“I’m sure there are people, including in the Democratic Party, who have relationships and feel uncomfortable about this,” Lehane said. But he added that the Bain argument is central to Obama because “the campaign is all about, ‘Who does the country trust?’”

Obama, a number of Democratic donors said, is already perceived as so anti-business by the financial services sector that his comments today won’t make much difference.

A veteran Democratic operative agreed and argued that Romneyland had been led down another rabbit hole trying to defend this bit of his curriculum vitae.

“This is not a good playing field for them,” the operative said.

Attacks on private equity have been effective in Romney’s past races — the unveiling of attacks featuring the bankrupt Bain-owned company Ampad seemed to catch Romneyland without a strong response, despite the fact that Ted Kennedy used it to great effect in their 1994 Senate race.

It’s also worked in other states. Dan Malloy won Connecticut in part thanks to blistering attacks on leveraged buyout manager Tom Foley, his Republican rival. The ads were effective and worked on a message that Foley and his business model were part of cheating the system.

That’s not quite how Chicago has delivered the Bain message, at least not prior to today. Instead, it was the pro-Obama super PAC Priorities USA Action that delivered a much more frontal TV ad that essentially said the Romney method was “heads, I win; tails, workers lose.”

The only way that Chicago can drive this narrative is through a slew of paid advertising, one Democratic strategist said.

“If you put real points behind this message, if delivered effectively, it will have currency,” the operative said. “If you don’t put real points behind it, … then I think it [muddies] it up. I don’t get the sense that it’s breaking through in the free press.”

There’s another concern for Obama, the operative said, which is that “it’s very hard to drive a negative message for five months.”

“The Obama campaign strategy is to drive a wedge between Mitt Romney and the middle class, first through his record in business and second through issues like the Buffett rule,” Grunwald said. “Yes, they will have to eventually put millions of dollars behind that strategy, but it doesn’t have to happen in the next week or two — from what I can tell, this is the focus of their whole campaign.”